1. Field of the Invention
This invention relates to a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
As is disclosed in Japanese Unexamined Patent Publications Nos. 55(1980)-112829 and 56(1981)-148636, there has been known a fuel injection system in which a fuel injection valve is provided in each of the intake pipes connected to the respective cylinders and fuel is injected from the fuel injection valve in synchronization with the intake stroke of the corresponding cylinder. The fuel injection system of this type is advantageous in that an optimal amount of fuel can be fed to each cylinder even if there is a variation in characteristics among the cylinders and accordingly the engine can be operated efficiently. Particularly in the system disclosed in the latter Japanese Unexamined Patent Publication, so-called stratified charge is used. That is, fuel is injected in the latter half of the intake stroke so that the fuel and air are not mixed uniformly in the cylinder but are in layers, the air-fuel mixture in the upper portion of the cylinder (where a spark plug is positioned) being rich and that in the lower portion of the cylinder being lean. Though the air-fuel ratio of the mixture surrounding the spark plug must be sufficient to be ignited by the spark plug, the lower layer may be solely of air or of very lean mixture. Therefore, the air-fuel ratio of the overall mixture may be very lean, whereby the specific fuel consumption can be substantially reduced and at the same time, unburned components such as CO and NOx can be reduced.
Because, it is difficult to obtain satisfactory response of the engine when the engine is to be accelerated due to delay in detection by the detecting system, the amount of fuel fed to each cylinder has been increased during acceleration of the engine. In order to apply this system to the fuel injection systems described above in which fuel is injected separately for each cylinder, the manner of increasing the amount of fuel is very important for maintaining high operating efficiency of the engine. Particularly in the latter fuel injection system in which stratified charge is accomplished, if the manner of increasing fuel is not proper, the air-fuel ratio deviates from a target value so that the air-fuel mixture becomes too rich and/or the engine stalls, and thus the advantage of the system that the air-fuel mixture fed to each cylinder may be lean is lost.